Photomasks are used extensively in the fabrication of integrated circuits on semiconductor wafers. Standard photomasks include a patterned light reflecting opaque layer or film on a transparent substrate. A metal, such as chromium, having a thickness on the order of about 1000 .ANG. is often used as the opaque layer or film. Nickel and aluminum also are used. Quartz is often used as the transparent substrate, though materials such as glass and sapphire can also be used.
Phase shift photomasks may include, in addition to patterned chromium on quartz, means to change the phase of light near the chromium edge, such as an etched region in the quartz or a transparent layer on the quartz near a chromium edge. Phase shift photomasks can also include, instead of a totally opaque material, an absorbing material (i.e., an attenuator) that permits transmission of a small fraction of the light.
The fabrication of photomasks usually involves a blanket deposition of a thin layer or film of chromium on the quartz substrate. The chromium is then coated with photoresist, which is exposed with a high resolution technique, such as an electron beam, and developed to form the desired pattern in the resist. This pattern is then transferred into the chromium by etching, thereby leaving opaque and transparent regions on the mask.
The photomask manufacturing process often results in imperfections, so that defects frequently are encountered during inspection of the photomasks. Defects are categorized as either "clear defects," or "opaque defects." Clear defects are regions that are designed to have the chromium but which actually do not have chromium. FIGS. 1a through 1f illustrate six of the most common defects seen on photolithography masks. FIG. 1a shows a mask 20 having an opaque bridge 21 between chromium lines 22a and 22b on a quartz substrate 24. FIG. 1b shows an opaque extension 26 extending from a chromium line 22a into a transparent region 27 between chromium lines 22a and 22b on a quartz substrate 24. FIG 1c shows an isolated opaque spot 28 in transparent region 27 between chromium lines 22a and 22b on a quartz substrate 24. FIG. 1d shows a pinhole 30 in a chromium line 22a on a quartz substrate 24. FIG. 1e shows a clear extension 32 into a chromium line 22a on a quartz substrate 24. FIG. 1f shows a clear bridge 34 extending across a chromium line 22a on a quartz substrate 24.
Masks having sufficiently large structures can be repaired using a technique described in U.S. Pat. No. 3,748,975, to Tarabocchia, in which photoresist is applied to the mask and exposed in a rectangular region including the defect to either open up a window for etching opaque defects away or to leave a region of tinted negative photoresist to fill in a clear defect. However, for opaque and clear defects that are attached to metal lines, normal process variations in the exposure, develop, and etch steps are too great to remove just the required amount of defect and provide a line having the desired shape within the tolerance required of present and future masks. For example, masks are currently being generated with lines having a width of 0.3 mm that typically have a tolerance in that width of about ten percent or, more preferably, about five percent. This precision is not achievable with standard photolithographic techniques and the repair is likely either to leave unwanted material on the mask or to remove wanted material from the adjacent opaque region. Thus, present masks have been repaired using highly focused beams of photons or ions.
More specifically, opaque defect repair currently involves laser evaporation or ablation or focused ion beam (FIB) sputtering of the unwanted chromium in defect regions such as opaque bridge 21 of FIG. 1a, opaque extension 26 of FIG. 1b, and opaque spot 28 of FIG. 1c. However, as with photolithographic processes, because the resolution of a laser is limited, if the opaque defect is connected to an adjacent chromium line as in FIGS. 1a and 1b, laser ablation may damage that adjacent line, removing some wanted chromium from the line. In addition, because a great deal of thermal energy is transmitted with the laser beam, the laser ablation step not only melts and vaporizes the unwanted metal defect region, but also damages and removes a layer of quartz underlying and adjacent the opaque defect, producing roughness in the quartz, as described in a paper "Effect of Laser Mask Repair Induced Residue and Quartz Damage in Sub-half-micron DUV Wafer Process," by Peiyang Yan et al., Proceedings of the 15th Annual Symposium on Photomask Technology and Management, Sep. 20-22, 1995, SPIE Volume 2621, pp. 158-166. This damaged region of the quartz is also responsible for reduced transmission and altered phase of transmitted light.
As an alternative to laser ablation, FIB offers a controlled process for sputtering a small region of unwanted material. The ion beam can be focused to a much smaller size than the laser beam. In addition, the ion beam physically sputters material, transmitting very little thermal energy to the mask. Thus, the quartz is not pitted. However, there are a number of problems that limit the use of FIB for mask repair. First, because a mask is formed on a quartz substrate which is an insulating material, the ion beam rapidly charges the surface and both the ability to aim subsequent ions and to use the ion beam to image the results is degraded. Second, while an opaque defect is being removed, quartz at the edge of the defect is attacked at the same rate and the result is a "river bed" or trench of damaged quartz around the defect, the quartz in this region having altered transmission and phase. Third, the focused ion beam species is typically gallium and gallium has been found implanted into the quartz when the opaque defect is removed, causing transmission losses. Fourth, the sputtering of material by the ion beam leads to ejection of material in all directions and some of this ejected material comes to rest on adjacent edges as described in an article by J. M. E. Harper et al., "Method for Improving Resolution of Focused Ion Beam Mask Repair Process," IBM TDB, n10a, March, 1991, pp. 174-176.
Clear defects can be repaired using a beam stimulated deposition process such as laser deposition or FIB induced deposition. These processes typically involve the decomposition of volatile organometallic complexes for which decomposition can be induced by the beam. Mask repair systems for laser deposition and FIB deposition have been commercially available for many years and the processes are well known in the art. However, a thin halo of deposited material is found adjacent the laser repaired region. The halo is the result of deposition along the periphery of the laser spot where there is a low, but non-negligible, light intensity. Thus, the edges of laser deposited material are difficult to control and trimming is usually required. This trimming step introduces the same kinds of problems and defects which occur when removing other opaque defects.
Similarly, while FIB deposition to repair clear defects is more controllable than laser deposition, because the ion beam current profile also has a long tail which extends well beyond the nominal beam diameter, material is deposited in a relatively large area surrounding the intended deposit, as described in an article "Elimination of Excess Material During Focused Ion Beam Induced Deposition", by P. G. Blauner, IBM Technical Disclosure Bulletin, v 39, n1, January, 1996, pp. 287-290. This peripheral film is a fraction of the thickness of the intended deposit but often must be removed to avoid degradation of transmission in surrounding clear regions. Removal of the peripheral film, or "halo", requires an extra step which adds to the complexity and time required for the repair process and introduces another possibility for damaging the quartz or otherwise introducing defects. In addition, because the quartz substrate charges during ion beam processing, the ion beam can be deflected and the repair patch, therefore, will not be located where expected.
One proposed clear defect repair technique, described in U.S. Pat. No. 4,200,668 to Segal et al., provides for repairing pinholes in the metal of a photomask by depositing resist on the mask, opening a window exclusively in the region of the pinhole by burning through the resist in that region with a laser, etching in the window to remove additional metal, thereby widening the pinhole and providing a more adhesive surface, depositing an opaque material on the entire surface and then lifting off the opaque material on the resist leaving it in the window adhering to the adjacent metal surrounding the pinhole. While this repair process is suitable for isolated pinhole defects, it is not adequate for clear defects at the edge of a metal line that require accurate reshaping and aligning, such as the clear extension and clear bridge of FIGS. 1e and 1f.